Poikilothermic organisms regulate the fluidity of their cellular membranes in response to temperature change through a complex homeostatic process known as homeoviscous adaptation. This phenomenon, evident across the phylogenetic spectrum, usually involves temperature-dependent alteration of membrane lipid composition. In the mold Neurospora crassa, a novel temperature-induced change in plasma membrane sphingolipids has been observed, which appears to be associated with a mechanism for the maintanence of fluid membranes at low temperature. This suggests that modulation of sphingolipid metabolism is a component of homeoviscous adaptation in the organism. Little is known about the integration of sphingolipid metabolism with other pathways involved in the adaptive process, and even less about interactions between sphingolipids and other lipid and protein components in the fungal plasma membrane. This research proposal addresses these points and proposes to extend our investigations to elucidate the role of sphingolipids in plasma membrane function in Neurospora. In particular, this research will address the following points: 1. Effects of temperature acclimation on sphingolipid biosynthesis, and the temporal relationship between sphingolipid and phospholipid alterations in the plasma membrane. 2. Effects of temperature-dependent lipid modifications on molecular dynamics in the plasma membrane. 3. Effects of inhibitors of sphingolipid biosynthesis on the lipid composition, fluidity and function of the plasma membrane. 4. Transbilayer distribution of sphingolipids in the Neurospora plasma membrane, and the effects of lipid compositional change on molecular dynamics in each leaflet of the membrane.